1. Field of the Invention
This invention relates to a vehicle differential gear device, a vehicle combined differential gear device and a vehicle differential case, each of which comprises plural pinion gears to receive a rotational driving force from the driving side and a pair of side gears to engage with the plural pinion gears.
2. Description of the Related Art
FIG. 46 shows a conventional vehicle differential gear device (e.g., Japanese utility model registration No. 2520728 (FIG. 1)). As shown in FIG. 46, the vehicle differential gear device 461 comprises a differential case 462 to be rotated by engine torque, side gears 463L, 463R that are disposed parallel to each other along a rotation axis line, and pinion gears 464 to engage with the side gears 463L, 463R.
The differential case 462 comprises a pinion gear insertion hole 465 and axle shaft insertion holes 466L, 466R, wherein their axis lines are at right angles to each other. The differential case 462 further comprises a concave groove 468 that is provided on the inside of the pinion gear insertion hole 465 to allow a snap ring 467 to be fitted inside there.
The side gears 463L, 463R are formed with bottomless and cylindrical bevel gears that comprise bosses 469L, 469R and gear portions 470L, 470R. They are disposed movably in the direction of the rotation axis line of the differential case 462 and are installed rotatably in the differential case 462 while keeping the bosses 469L, 469R placed in the axle shaft insertion holes 466L, 466R. Inside the side gears 463L, 463R, a part of axle shafts 471L, 471R is positioned and splined in the axle shaft insertion holes 466L, 466R. Annular sliding members 472L, 472R are disposed on the periphery of the bosses 469L, 469R, and between the gear portions 470L, 470R (backside thereof) of the side gears 463L, 463R and the periphery of the inside opening of the axle shaft insertion holes 466L, 466R.
The pinion gear 464 is formed with a bottomless and cylindrical gear, retained by a pinion gear retaining plate 473 disposed between the snap ring 467 and the pinion gear 464, and rotatably installed in the pinion gear insertion hole 465. A pinion gear shaft 474 is attached to the center of the pinion gear 464 to prevent the slanting of the gear.
The vehicle differential gear device 461 is assembled such that the sliding members 472L, 472R and the side gears 463L, 463R are first inserted in the pinion gear insertion hole 465 to be installed in the differential case 462, the pinion gear 464 is then inserted in the pinion gear insertion hole 465 to engage with the side gears 463L, 463R and to be fitted to the pinion gear shaft 474 previously placed in the differential case 462, the pinion gear retaining plate 473 is disposed on the back side of the pinion gear 464, and the snap ring 467 is fitted into the concave groove 468.
However, the conventional vehicle differential gear device of Japanese utility model registration No. 2520728 has problems as described below (1) to (4).
(1) Since the pinion gear shaft is attached to the center of the pinion gear, the outer diameter of the pinion gear needs to be relatively large. Therefore, the outer diameter of the side gear is reduced to cause an increase in rotation backlash tan−1(b/r) which is determined by a backlash b defined between the pinion gear and the side gear and a radius r of the side gear. As a result, the transmission of driving force will deteriorate.
(2) As the outer diameter of the pinion gear increases, distance between the side gears is extended. Thus, the differential case cannot be downsized,
(3) As the outer diameter of the pinion gear increases, the sliding diameter of the pinion gear increases. Due to expansion caused by the sliding of the pinion gear, a proper clearance between the pinion gear and the pinion gear insertion hole becomes difficult to secure As a result, the pinion gear may be seized.
(4) As the outer diameter of the pinion gear increases, the mass of the pinion gear increases so that the pinion gear can be influenced by centrifugal force. This allows a variation in differential limiting force generated at a pinion gear support portion (not shown) located outside of the pinion gear. Thus, the vehicle differential gear device cannot have a stable performance.